PTB48540AAH_14 Datasheet, PDF (6/13 Pages) Texas Instruments – 10-W Power-Over-Ethernet Isolated Power Module Assembly

Not Recommended for New Designs
Application Notes continued
PTB48540 Series
Table 1-1; PD Class Programming Resistance
PD
Class
0
1
2
3
4
Usage
Default
Optional
Optional
Optional
Not Allowed
PD Demand (watts)
Min
Max
0.44
12.95
0.44
3.84
3.84
6.49
6.49
12.95
— Future Use —
Class Program
Resistance
4.42 kΩ
950 Ω
543 Ω
360 Ω
252 Ω
External
Resistor
None
1.21 kΩ
619 Ω
392 Ω
267 Ω
for the different types of PD class defined in the IEEE
802.3af Standard. The power modules support the PD
classification protocol with a default ‘Class 0’ signature.
‘Class 0’ corresponds to a 4.42-kΩ programming resis-
tance, which is set by an internal resistor located between
the ‘Class A’ (pin 12) and ‘Class B’ (pin 11) terminals. By
placing an external resistor across the Class A/B pins (in
parallel with the internal 4.42-kΩ resistor) the power
module can be made to communicate one of the alternate
classifications to the PSE. Consult Table 1-1 for the
external resistance values.
Under-Voltage Lockout
The UVLO prevents the internal DC/DC converter from
seeing an input voltage until the voltage applied to either
the ‘Data Line A/B’ or ‘Spare Line A/B’ pair of Ethernet
connections approaches 42 V. The UVLO threshold
correlates to a voltage between the ‘Detect B’ (pin 6) and
‘Class B’ (pin 11) terminals of approximately 39.3 V. Only
after the voltage applied from the Ethernet is above the
UVLO threshold is the module’s internal bus voltage
allowed to rise. The internal bus powers the DC/DC
converter and can be measured between the ‘Detect B’
(pin 6) and ‘Input Ref’ (pin 4) terminals.
Input Capacitance
In accordance with the IEEE 802.3af Standard, the
PTB48540 power modules provide an input capacitance
of 0.1 µF to the PSE when communicating the required
detection and classification signatures. Once fully pow-
ered (V_in ≥42 V), the PSE will see the combined input
capacitance of the EMI filter and DC/DC converter;
approximately 7 µF. This capacitance is sufficient to
operate the module’s internal DC/DC converter, and
satisfies the 5 µF minimum capacitance required by the
IEEE 802.3af Standard. For improved hold-up capability,
this input capacitance can be increased with an external
capacitor. Connect the anode of the external capacitor
to ‘Detect B’ (pin 6), and the cathode to the ‘Input Ref’
(pin 4). During power up the power interface IC limits
the inrush current for charging the input capacitance.
Additional capacitance increases the power dissipation in
the IC. For this reason the maximum recommended value
of external capacitance is 220 µF (100-V electrolytic).
Startup
Startup of the module in a PoE application consists of a
complex process of handshaking states between the module
and PSE. During the PD detection state the PSE uses a
low voltage (<10 V) to detect the module’s “valid device”
resistance signature. This is followed by the classification
detection state where the PSE applies a voltage of 15 V
to 20 V to detect the module’s PD class. The PSE con-
tinues to raise the input voltage, but the input voltage to
the internal DC/DC converter is held at zero until the
voltage from the PSE approaches 42 V. At an input voltage
of 42 V or higher, the module’s power interface IC allows
the internal bus voltage to rise using a limited amount of
inrush current. Approximately 50 ms after the DC/DC
converter input filter capacitors are fully charged, the
module is able to produce a regulated output voltage.
Converter Over-Current Protection
The internal DC/DC converter has inherent protection
against an output load fault. Whenever its load current
exceeds the over-current protection threshold (see speci-
fication table) the converter momentarily turns its output
off. After a short period (<100 ms), the regulator will
attempt to power up again by executing a soft-start power
up. The converter will continue in a successive cycle of
shutdown, followed by soft-start power up until the load
fault is removed.
When the DC/DC converter is powered from just the
Ethernet source, a load current above its rated output
(but below its over-current threshold) will likely activate
the over-current protection offered by the power interface
IC. This is especially at input voltages lower than 48 V.
Ethernet Over-Current Protection
Protection is also provided for the Ethernet power source
equipment (PSE). In event of a fault across the module’s
internal bus, the onboard power interface IC limits the
maximum current that may be drawn from the PSE to
no less than 405 mA. This prevents the DC/DC con-
verter from drawing excessive input current and also
safeguards against external faults that may occur across
the ‘Detect B’ (pin 6) and ‘Input Ref’ (pin 4) terminals.
Note: These terminals can be used to add capacitance to the
module’s internal bus.
Load faults applied to the DC/DC converter’s output
will most often trigger the power interface IC’s protec-
tion mechanism prior to activating the converter’s own
current limit threshold. In these instances the power
interface IC will completely shut down the module’s inter-
nal bus. This is a latched condition. It is reset by the PoE
source when it attempts another power-up cycle after it
senses loss of the PD’s Maintain Power Signature (10 mA).
For technical support and further information visit http://power.ti.com